A vehicle powertrain typically includes an automatic transmission which may be, for example, a step-gear transmission or a continuously variable transmission (CVT). It may be desirable to promote fuel economy in a vehicle by controlling the transmission of the vehicle accordingly. For example, a CVT can be used to reduce engine speed (engine RPM, or ERPM) for a given axle power. In other words, the engine speed can be optimized for economy and the CVT varies the speed of the vehicle. In a vehicle in which a CVT is utilized, brake-specific fuel consumption (BSFC) can be reduced due to lower pumping losses and lower rubbing friction.
There are limitations, however, to the extent of ERPM reduction that can be implemented during driving. For example, excessive ERPM reduction could increase noise and vibration of the driveline and/or vehicle body and could degrade the driving experience. Combustion detonation, or “knock”, might also occur. Knock can occur for a given ERPM or load due to high ambient air or engine temperatures. Poor fuel quality and low fuel octane also can cause knock. As ERPM is reduced during vehicle operation, knock can arise even before the ERPM is reduced to levels where noise or vibration might occur. Vehicles having step-gear transmissions also may be subject to knock.
CVTs are usually calibrated conservatively, i.e., to avoid knock-sensitive operating conditions. If a CVT is calibrated aggressively, for example, to promote fuel economy and knock is detected, an engine control module would typically retard spark in response. Although knock would be reduced by this response, it is likely that fuel efficiency and driver satisfaction also would be reduced.